The invention relates generally to apparatus and procedures for making rapid determination of the strength of metal materials from hardness evaluation of the materials, and particularly to the elimination of errors and uncertainties in conventional hardness testing techniques that relate yield and/or tensile strength to hardness.
Hardness tests are commonly employed in quality assurance testing to indicate material strength. Typically, correlations for particular alloy-temper combinations are expressed as equations relating strength (S) to an observed hardness member (H) in the form of EQU S=a+b.times.H (1)
where the values of a and b depend on the hardness test scaleselected.
Small scale tests, such as hardness tests, are convenient and less expensive than tests involving machined or otherwise especially prepared specimens for tensile testing. However, such convenience has led to a bewildering variety of hardness test scales. Generically, these can be divided into scales using ball-shaped indentors such as the Brinell and Rockwell indentors or diamond-shaped indentors such as Vickers or Knoop.
One significant disadvantage of the Rockwell test lies in the large variety of scales, i.e., no single scale adequetely spans the whole range of interest for aluminum alloys, for example. Each scale has an optimal application in terms of material strength (temper) and minimum thickness necessary to avoid the so-called anvil effect, which effect involves variations in hardness readings due to the hardness of the structure supporting the specimen.
Nonetheless, for approximate practical purpose, the Rockwell scales can be ranked in terms of their severity of loading by dividing the applied measured load (L) by the diameter of the ball squared (D.sup.2), which is the ratio L/D.sup.2. If the reading on a particular scale is above say 100 (a hardness number), one has to then provide a scale with lower numerical readings; if the normal scale produces low readings, such as values below 20, a scale with a lower L/D.sup.2 value is needed. This often necessitates changing scales in the midst of an investigation, which further complicates the use of a strength hardness equation such as equation (1) above.
Rockwell and other hardness tests, in addition, do not provide unambiguous predictions of yield or tensile strength of materials tested. This is the result of the influence of work hardening that occurs in the process of making the impressions. This influence can be understood by expressing hardness as a flow stress and releating it to yield or tensile strength through the well-known, constitutive stress-strain relationship.
The general conclusion from such analyses has been that one must know the work-hardening coefficient and the degree of strain imparted by the indentation process to predict yield or tensile strength from a hardness number.
An empirical way of circumventing the need for such complete knowledge was proposed in an article entitled "Estimating Yield Strength from Hardness Data" by Robert A. George, Subimal Dinda and Arthur S. Kasper, published in the May 1976 issue of Metal Progress, the authors use the basic relationship between applied load (L), indentor diameter (D) and the impression (d) of the form EQU L/d.sup.2 =A (d/D).sup.m (2)
to predict yield strengths of various steels. (A and m are empirical constants.) This work correlated yield strength with the constant A in the form of a regression equation, i.e., EQU ys (ksi)=0.325A (3)
with A being determined from nomographs of A versus a particular Rockwell number.